1. Field of the Invention
The present invention relates to an ignition coil used in an internal combustion engine.
2. Description of the Related Art
Heretofore, in a distributorless ignition (DLI) type internal combustion engine, for example, a circuit, in which a spark plug and an ignition coil are connected directly with each other, has been proposed (JP-A-63-132411, JP-A-64-8580, etc.).
An iron core for use in such an ignition coil is composed of an I-shaped first core 101, around which a primary coil (not shown in the figure) and a secondary coil (not shown in the figure) are wound, and a U-shaped second core 102 forming a closed magnetic path in conjunction with the first core 101, as shown in FIG. 19. Further, another ignition coil has been proposed in which a permanent magnet 103 is disposed for the purpose of increasing magnetic energy stored in the iron core so as to increase an induced electromotive force of the secondary coil by making (biasing) magnetic flux pass through the closed magnetic path.
Although such an ignition coil as described above is disposed between two banks of an internal combustion engine in order to connect it directly with a spark plug, it may be readily thought of to dispose the ignition coil within a plug tube 104 so as to be incorporated with the plug tube 104 which is made of iron to serve as a mounting hole for mounting a spark plug disposed between the two banks.
However, in the case of an ignition coil disposed in the plug tube 104, not only magnetic flux .PHI..sub.c passes through the closed magnetic path formed by the first core 101 and the second core 102, but also leakage flux .PHI..sub.L passes through the plug tube 104. Therefore, with the ignition coil described above, there is a possibility that an eddy current is produced in the plug tube 104, because the leakage magnetic flux .PHI..sub.c passes through the plug tube 104.
When such an eddy current is produced in the plug tube 104 in this way, the magnetic energy stored in the iron core decreases, resulting in electric power loss (eddy current loss).
For this reason, since an induced electromotive force generated in the secondary coil of the above-described ignition coil decreases significantly, such an ignition system has a drawback that a high voltage (a generated voltage) applied to the spark plug is significantly lowered (refer to the graph indicated in FIG. 20). In the graph indicated in FIG. 20, I.sub.1 =6.5 A represents the value of a current flowing through the primary coil. Furthermore, "a" indicates the case of an ignition coil disposed where there were no parts of an internal combustion engine made of a conductive material located in the neighborhood of the place. "b" indicates the case of an ignition coil disposed in the plug tube. In the experiments for making the comparison, an ignition coil was used in which a permanent magnet was disposed in the closed magnetic path of the ignition coil.